In developing a method of producing polyurethane prepolymers, curable by atmospheric humidity, there are several important requirements which must be met if the new process is to be commercially feasible. First, a general preparatory method should have the flexibility of producing prepolymers with varying physical properties for various applications such as sealants, elastomers and foams. This flexibility is usually achieved by varying the molecular weights and weight ratios of the diols and triols. Second, it is preferable to form a prepolymer with a relatively high degree of conversion of isocyanate to urethane because this reduces the time required to cure the prepolymer. Third, for obvious economic reasons, the preparation should be fast and should not require expensive, special purpose equipment.
The prior art teaches several methods of forming moisture-curable, polyurethane prepolymers from diisocyanates, polyols and suitable catalysts. However, none of the techniques taught or suggested by the prior art meet all of the above goals.
In the past, developmental efforts have been directed at the problems of maintaining control of the polymerization reaction and of producing a uniform prepolymer. These problems were believed to be caused by the highly exothermic nature of the polymerization reaction and the positive temperature coefficient of the reaction rate constants. These factors indicate that the reaction may become autocatalytic.
Kinetically, the polymerization reaction is of the second order and, therefore, on a macroscale the rate of reaction is dependent on the degree of mixing. In addition, the polymerization is a very fast exothermic reaction and the contact between the reactants must be uniform throughout the batch to prevent the formation of undesirable gel particles. These are small beads which contain high molecular weight and/or a highly branched polymer and are formed when the stirring of the reacting mixture is not sufficient to maintain a uniform dispersion of the polymerizing reactive sites. If the highly branched polymer segments, having a multitude of reacting sites, are allowed to cluster and generate localized hot spots in which the rate of polymerization is very high, then they will develop into gel particles. The tendency to form gel particles is increased as the degree of molecular branching in the triol is increased, and can be significantly reduced by eliminating the triol. However, this causes a significant loss in the tensile strength and modulus of the cured prepolymer.
Prior to this invention, it was believed that the only reasonable approach to the problem created by the polymerization exotherm was to greatly reduce the rate of polymerization. In general, this was accomplished by either using a noncatalyzed prereaction step in which the isocyanate was slowly reacted with some of the polyol, usually the diol, or by omitting the triol completely, which would eliminate any crosslinking reactions.
The prior art teaches at least three specific techniques of managing this problem. First, there is a widely used two-step method; during the first step of this process the diisocyanate and a diol are mixed, without a catalyst, and reacted at about 180.degree. F. until the mixture is free of hydroxyl (OH) groups and contains about 3.5% of unreacted or free isocyanate (NCO) groups. This reaction takes about three to four hours and, therefore, is a severe bottleneck in any production application. The second step of this reaction is to add a branched triol and a tin-based catalyst to the above reaction product, and stir vigorously. The polymerization reaction is then continued at about 140.degree. F. until the free isocyanate (NCO) is reduced to about 1.3%; this step requires an additional 30 to 60 minutes. The prereaction serves to increase the viscosity of the subsequent polymerizing mixture and this physically slows the reaction by reducing the rate at which the reactive hydroxyl (OH) and isocyanate (NCO) functionality come into contact with each other.
In the second prior art approach, the problem is avoided by using only a linear diol and then stopping the catalyzed reaction at moderately low degrees of conversion, as indicated by the ratio of isocyanate (NCO) group to hydroxyl (OH) groups within the range of about 2:1 to about 4:1. Omitting the triol makes the prepolymer unsuitable for many applications, but this modification effectively controls the polymerization exotherm and reduces the required reaction time to about 30 minutes.
The third prior art technique of managing the problem is to form a prepolymer with a relatively low degree of conversion and then depend on the subsequent prepolymer curing reaction to complete the polymerization. In one specific method the ratio of isocyanate groups to hydroxyl group in the prepolymer is in the range of about 4:1 to about 10:1. The subsequent curing step is a time consuming process.